This invention relates to a method for producing a corrosion and pitting resistant austenitic stainless steel in heavy section sizes and as welded articles. More particularly, the invention relates to methods of producing such steels having higher nitrogen contents which produce a steel substantially free of second phase precipitation.
It is known that stainless steels have corrosion resistance properties which make them useful in various corrosive environments. Service in highly corrosive media requires steels especially alloyed to withstand the corrosive effects. Chloride pitting and crevice corrosion are severe forms of corrosion which result from metal contact with the chloride ion in corrosive environments such as sea water and certain chemical processing industry media. To be resistant to pitting corrosion, certain austenitic stainless steels have been developed having relatively high chromium and molybdenum levels such as described in Bieber et al U.S. Pat. 3,547,625, issued Dec. 15, 1970. Other examples of austenitic stainless steels containing high levels of molybdenum and chromium are U.S. Pat. Nos. 3,726,668; 3,716,353; and 3,129,120. Such stainless steels with a relatively high molybdenum content sometimes exhibit poor hot workability.
Alloying additions have been used to improve hot workability. U.S. Pat. No. 4,007,038, issued Feb. 8, 1977, describes a high molybdenum-containing alloy with good pitting resistance and good hot workability by virtue of the addition of critical amounts of both calcium and cerium and which has found commercial acceptance. A chromium-nickel-molybdenum austenitic stainless steel having enhanced corrosion resistance and hot workability is disclosed in U.S. Pat. No. 4,421,557, issued Dec. 20, 1983, by additions of the rare earth element lanthanum singly or in combination with nitrogen of 0.12 to 0.5%. Nitrogen is a known austenitizing element which is described in the literature as being useful for reducing the sigma phase and by increasing the time to precipitate the chi phase in a 17% Cr--13% Ni--5% Mo stainless steel.
Such high molybdenum-containing austenitic stainless steels are typically used in thin gauges, such as 0.065 inch (1.65 mm) or less in strip form or as tubing and have excellent corrosion properties. As the gauge, section thickness or shape of the article increases, there is a severe deterioration of corrosion properties due to the development of intermetallic compounds (second phases), such as sigma and chi. Such phases develop upon cooling from a solution annealing temperature or from welding temperatures. Such precipitation of second phases has deterred the commercial selection and of such material in sizes other than thin strip or thin-walled tubing.
Generally, as the presence of the sigma and chi phases are detrimental to corrosion resistance, special heat treatments are necessary to attempt to eliminate the sigma phase. For example, for alloys nominally 25 Ni--20 Cr--6 Mo, described in the above U.S. Pat. No. 4,007,038, such heat treatments require heating in excess of 2000.degree. F. (1093.degree. C.) or more followed by a rapid cooling. As a practical matter for commercial production, such alloys are generally heated in excess of 2150.degree. F. (1177.degree. C.). A practical problem of such requirements is that such practices restrict the useful equipment as, well as, restrict the size or shape of the articles made from such alloys. For example, some applications often require heavy gauge support products, such as plate, as well as light gauge weldable tubing, such as condenser tubing After assembly by welding, the size and shape of the assembled equipment may prevent use of a final heat treatment or if capable of a heat treatment, the size and shape may severely limit the ability to cool rapidly from the heat treatment or weld temperature. The cooling rates of heavier sections are slower than those of thinner sections when water quenched or air cooled.
What is needed is a method of producing an austenitic stainless steel alloy in heavier plate sections which are weldable and which has the same corrosion resistance as thin strip. It is also an object to produce such stainless steel articles without the need for extraordinary heat treating and cooling steps. It is a further object to modify the kinetics of the precipitation of the sigma phase in the Cr--Ni--Mo alloys in order to reduce the amount of second phase precipitated during cooling from the annealing and welding temperatures.